I saw Jerusalem in New York in May. It's a great play, but Mark Rylance's landmark performance as Johnny "Rooster" Byron raised it, and I was completely gobsmacked. It was tightly tuned, like a Ferrari, packed with power, of wit, humour, warmth, charisma. It's so mercurial, it could switch in a second, but Rylance never once came out of character – which of course you shouldn't, but a lot of people do.

Jerusalem is a comic vision of life in contemporary rural England. Written by Jez Butterworth, it's partly a lament for the erosion of that country life, but it also rebuffs the antiseptic and modern world we live in now. Rylance plays a man of Romany heritage, a former daredevil motorcyclist who lives in a caravan in the woods; an individual who sticks to his guns, avoids taking responsibility and goes against the establishment. He's a genius with the detail, dynamic and unpredictable, and always making clear Rooster's underlying motivations, such as his drug use and his Peter Pan mentality. As I watched, I wondered, was the part written this way, or did he add loads of bits in here and there?

He's a great lead but he never overshadowed the play. When the action focused on other characters, he just sat there, rolling a fag, but always in it, always reacting. I couldn't take my eyes off him.

Every moment of Rylance's performance was rise and fall, dark and light, the full gamut of colour. He looked like a pikey, with his vest on, all beered up; then he got dressed in a suit and became quite serious with the council; and the gentleness he has with the son – his range is gargantuan.

At the end, when he's beating a drum, the rhythm made a kind of heartbeat in the theatre. I looked around at the rest of the audience and there were mouths wide open in awe and admiration. It's something I will never forget. Rylance is the type of actor you aspire to be like.